The present invention is directed generally to the implantation of artificial joint components and, more particularly, to computer assisted surgical implantation of artificial acetabular and femoral components during total hip replacement and revision procedures.
Total hip replacement (THR) or arthroplasty (THA) operations have been performed since the early 1960s to repair the acetabulum and the region surrounding it and to replace the hip components, such as the femoral head, that have degenerated. Currently, approximately 200,000 THR operations are performed annually in the United States alone, of which approximately 40,000 are redo procedures, otherwise known as revisions. The revisions become necessary due to a number of problems that may arise during the lifetime of the implanted components, such as dislocation, component wear and degradation, and loosening of the implant from the bone.
Dislocation of the femoral head from the acetabular component, or cup, is considered one of the most frequent early problems associated with THR, because of the sudden physical, and emotional, hardship brought on by the dislocation. The incidence of dislocation following the primary THR surgery is approximately 2-6% and the percentage is even higher for revisions. While dislocations can result from a variety of causes, such as soft tissue laxity and loosening of the implant, the most common cause is impingement of the femoral neck with either the rim of an acetabular cup implant, or the soft tissue or bone surrounding the implant. Impingement most frequently occurs as a result of the malposition of the acetabular cup component within the pelvis.
Some clinicians and researchers have found incidence of impingement and dislocations can be lessened if the cup is oriented specifically to provide for approximately 15.degree. of anteversion and 45.degree. of abduction; however, this incidence is also related to the surgical approach. For example, McCollum et al. cited a comparison of THAs reported in the orthopaedic literature that revealed a much higher incidence of dislocation in patients who had THAs with a posterolateral approach. McCollum, D. E. and W. J. Gray, "Dislocation after total hip arthroplasty (causes and prevention)", Clinical Orthopaedics and Related Research, Vol. 261, p. 159-170 (1990). McCollum's data showed that when the patient is placed in the lateral position for a posterolateral THA approach, the lumbar lordotic curve is flattened and the pelvis may be flexed as much as 35.degree.. If the cup was oriented at 15-20.degree. of flexion with respect to the longitudinal axis of the body, when the patient stood up and the postoperative lumbar lordosis was regained, the cup could be retroverted as much as 10.degree.-15.degree. resulting in an unstable cup placement. Lewinnek et al. performed a study taking into account the surgical approach utilized and found that the cases falling in the zone of 15.degree..+-.10.degree. of anteversion and 40.degree..+-.10.degree. of abduction have an instability rate of 1.5%, compared with a 6% instability rate for the cases falling outside this zone. Lewinnek G. E., et al., "Dislocation after total hip-replacement arthroplasties", Journal of Bone and Joint Surgery, Vol. 60-A, No.2, p. 217-220 (March 1978). The Lewinnek work essentially verifies that dislocations can be correlated with the extent of malpositioning, as would be expected. The study does not address other variables, such as implant design and the anatomy of the individual, both of which are known to greatly affect the performance of the implant.
The design of the implant significantly affects stability as well. A number of researchers have found that the head-to-neck ratio of the femoral component is the key factor of the implant impingement, see Amstutz H. C., et al., "Range of Motion Studies for Total Hip Replacements", Clinical Orthopaedics and Related Research Vol. 111, p. 124-130 (September 1975). Krushell et al. additionally found that certain long and extra long neck designs of modular implants can have an adverse effect on the range of motion. Krushell, R. J., Burke D. W., and Harris W. H., "Range of motion in contemporary total hip arthroplasty (the impact of modular head-neck components)", The Journal of Arthroplasty, Vol. 6, p. 97-101 (February 1991). Krushell et al. also found that an optimally oriented elevated-rim liner in an acetabular cup implant may improve the joint stability with respect to implant impingement. Krushell, R. J., Burke D. W., and Harris W. H., "Elevated-rim acetabular components: Effect on range of motion and stability in total hip arthroplasty", The Journal of Arthroplasty, Vol. 6 Supplement, p. 1-6, (October 1991). Cobb et al. have shown a statistically significant reduction of dislocations in the case of elevated-rim liners, compared to standard liners. Cobb T. K., Morrey B. F., Ilstrup D. M., "The elevated-rim acetabular liner in total hip arthroplasty: Relationship to postoperative dislocation", Journal of Bone and Joint Surgery, Vol 78-A, No. 1, p. 80-86, (January 1996). The two-year probability of dislocation was 2.19% for the elevated liner, compared with 3.85% for standard liner. Initial studies by Maxian et al. using a finite element model indicate that the contact stresses and therefore the polyethylene wear are not significantly increased for elevated rim liners; however, points of impingement and subsequent angles of dislocation for different liner designs are different as would be expected. Maxian T. A., et al. "Femoral head containment in total hip arthroplasty: Standard vs. extended lip liners", 42nd Annual meeting, Orthopaedic Research society, p. 420, Atlanta, Ga. (Feb. 19-22, 1996); and Maxian T. A., et al. "Finite element modeling of dislocation propensity in total hip arthroplasty", 42nd Annual meeting, Orthopaedic Research society, p. 259-64, Atlanta, Ga. (Feb. 19-22, 1996).
An equally important concern in evaluating the dislocation propensity of an implant are variations in individual anatomies. As a result of anatomical variations, there is no single optimal design and orientation of hip replacement components and surgical procedure to minimize the dislocation propensity of the implant. For example, the pelvis can assume different positions and orientations depending or whether an individual is lying supine (as during a CT-scan or routine X-rays), in the lateral decubitis position (as during surgery) or in critical positions during activities of normal daily living (like bending over to tie shoes or during normal gait). The relative position of the pelvis and leg when defining a "neutral" plane from which the angles of movement, anteversion, abduction, etc., are calculated will significantly influence the measured amount of motion permitted before impingement and dislocation occurs. Therefore, it is necessary to uniquely define both the neutral orientation of the femur relative to the pelvis for relevant positions and activities, and the relations between the femur with respect to the pelvis of the patient during each segment of leg motion.
Currently, most planning for acetabular implant placement and size selection is performed using acetate templates and a single anterior-posterior x-ray of the pelvis. Acetabular templating is most useful for determining the approximate size of the acetabular component; however, it is only of limited utility for positioning of the implant because the x-rays provide only a two dimensional image of the pelvis. Also, the variations in pelvic orientation can not be more fully considered as discussed above.
Intra-operative positioning devices currently used by surgeons attempt to align the acetabular component with respect to the sagittal and coronal planes of the patient. B. F. Morrey, editor, "Reconstructive Surgery of the Joints", chapter Joint Replacement Arthroplasty, pages 605-608, Churchill Livingston, 1996. These devices assume that the patient's pelvis and trunk are aligned in a known orientation, and do not take into account individual variations in a patient's anatomy or pelvic position on the operating room table. These types of positioners can lead to a wide discrepancy between the desired and actual implant placement, possibly resulting in reduced range of motion, impingement and subsequent dislocation.
Several attempts have been made to more precisely prepare the acetabular region for the implant components. U.S. Pat. No. 5,007,936 issued to Woolson is directed to establishing a reference plane through which the acetabulum can be reamed and generally prepared to receive the acetabular cup implant. The method provides for establishing the reference plane based on selecting three reference points, preferably the 12 o'clock position on the superior rim of the acetabulum and two other reference points, such as a point in the posterior rim and the inner wall, that are a known distance from the superior rim. The location of the superior rim is determined by performing a series of computed tomography (CT) scans that are concentrated near the superior rim and other reference locations in the acetabular region.
In the Woolson method, calculations are then performed to determine a plane in which the rim of the acetabular cup should be positioned to allow for a predetermined rotation of the femoral head in the cup. The distances between the points and the plane are calculated and an orientation jig is calibrated to define the plane when the jig is mounted on the reference points. During the surgical procedure, the surgeon must identify the 12 o'clock orientation of the superior rim and the reference points. In the preferred mode, the jig is fixed to the acetabulum by drilling a hole through the reference point on the inner wall of the acetabulum and affixing the jig to the acetabulum. The jig incorporates a drill guide to provide for reaming of the acetabulum in the selected plane.
A number of difficulties exist with the Woolson method. For example, the preferred method requires drilling a hole in the acetabulum. Also, visual recognition of the reference points must be required and precision placement on the jig on reference points is performed in a surgical setting. In addition, proper alignment of the reaming device does not ensure that the implant will be properly positioned, thereby establishing a more lengthy and costly procedure with no guarantees of better results. These problems may be a reason why the Woolson method has not gained widespread acceptance in the medical community.
In U.S. Pat. Nos. 5,251,127 and 5,305,203 issued to Raab, a computer-aided surgery apparatus is disclosed in which a reference jig is attached to a double self indexing screw, previously attached to the patient, to provide for a more consistent alignment of the cutting instruments similar to that of Woolson. However, unlike Woolson, Raab et al. employ a digitizer and a computer to determine and relate the orientation of the reference jig and the patient during surgery with the skeletal shapes determined by tomography.
Similarly, U.S. Pat. Nos. 5,086,401, 5,299,288 and 5,408,409 issued to Glassman et al. disclose an image directed surgical robotic system for reaming a human femur to accept a femoral stem and head implant using a robot cutter system. In the system, at least three locating pins are inserted in the femur and CT scans of the femur in the region containing the locating pins are performed. During the implanting procedure, the locating pins are identified on the patient, as discussed in col. 9, lines 19-68 of Glassman's '401 patent. The location of the pins during the surgery are used by a computer to transform CT scan coordinates into the robot cutter coordinates, which are used to guide the robot cutter during reaming operations.
While the Woolson, Raab and Glassman patents provide methods and apparatuses that further offer the potential for increased accuracy and consistency in the preparation of the acetabular region to receive implant components, there remain a number of difficulties with the procedures. A significant shortcoming of the methods and apparatuses is that when used for implanting components in a joint there are underlying assumptions that the proper position for the placement of the components in the joints has been determined and provided as input to the methods and apparatuses that are used to prepare the site. As such, the utility and benefit of the methods and apparatuses are based upon the correctness and quality of the implant position provided as input to the methods.
In addition, both the Raab and Glassman methods and apparatuses require that fiducial markers be attached to the patient prior to performing tomography of the patients. Following the tomography, the markers must either remain attached to the patient until the surgical procedure is performed or the markers must be reattached at the precise locations to allow the transformation of the tomographic data to the robotic coordinate system, either of which is undesirable and/or difficult in practice.
Thus, the need exists for apparatuses and methods which overcome, among others, the above-discussed problems so as to provide for the proper placement and implantation of the joint components to provide an improved range of motion and usage of the joint following joint reconstruction, replacement and revision surgery.